The invention relates to built-in test equipment and more particularly to passive fault detection circuitry incorporated into the electrical/electronic systems of an aircraft for detecting and identifying electrical faults in the systems' components, including elements such as temperature sensors, pressure sensors, valve actuating solenoids, and the like.
The electrical/electronic systems of a modern, commercial aircraft are made up of a large number of line replaceable units (LRUs). Each LRU is mounted on the aircraft in a manner that allows removal and replacement by maintenance crews, and each unit is interconnected, via cable wiring, with controllers, usually located in a central equipment bay. Thus, when a malfunction develops in one of the electrical/electronic systems, the first step is to evaluate the nature of the malfunction and identify the type and location of the causal fault. The fault must be localized to a particular LRU, which can then be replaced in order to restore the aircraft to service. The fault may be located either by means of BITE or by an evaluation of the symptoms of the fault by the flight crew and/or technicians from the ground maintenance crew. Typically, each LRU will contain one or more discrete components, the condition of which is critical to the proper operation of the associated LRU. For example, an LRU forming part of the environmental control system may contain one or more temperature sensors of a variable, thermosensitive resistance type. Another LRU of the same system may comprise a pressure sensor of a variable inductance type. These are merely examples of a few of the numerous LRUs that collectively form the environmental control system. A fault in one of these temperature or pressure sensors, or other component, will necessitate the replacement of the associated LRU. Each of the other electrical/electronic systems of the aircraft is similarly organized into numerous and various LRUs.
One of the most frequent kinds of fault occurs when a discrete electrical component constituting an LRU becomes damaged in a way that causes such component to have an open impedance. This type of fault may be detected and located in one of several ways. If the fault is a steady one, then a ground crew technician will, either upon his own discovery of the fault, or in response to a complaint from the flight crew, check out the various possible reasons for the malfunction using ground test equipment. For example, the impedance of several possible malfunctioning LRUs may be checked by measuring the impedance across the interconnect wiring that extends to each LRU component. In many instances, the individual LRU components are not readily accessible, being located at diverse points throughout the aircraft such that they are not easily reached by the ground crew, and the impedance check is therefore usually performed by testing the electrical condition of the component as such impedance appears at one or more conveniently and centrally located terminals in the electrical/electronic equipment bay. Having localized the system failure to the circuitry of a remote LRU, the technician will then go to the location of the remote LRU in the wing, empennage, engine installation, wheel well or other appropriate body location. There he will remove access panels if required to get to the LRU, disconnect the wiring connector, and test continuity of the wire run from the equipment bay to the LRU as well as testing the internal continuity of the LRU through the mating plug.
Alternatively, BITE systems are available for automatically monitoring the impedance of the component and generating a fault indicating signal for alerting the maintenance crews to the existence and location of the fault. As in the case of the above described, initial manual impedance check, the BITE impedance measurement is also performed at the centrally located terminals.
In the past, the foregoing fault detection and locating techniques have not proved to be entirely successful in terms of providing quick, cost effective correction of electrical/electronic system faults. The drawbacks of such techniques are particularly acute in the case of intermittent faults, namely faults that occur intermittently during flight, but do not reoccur when the electrical/electronic systems are checked out by the technicians while the aircraft is on the ground. When the ground crew has been alerted to such an intermittent fault, a common practice is to replace the LRU to which the fault has been isolated, even though the fault does not reoccur during ground testing. Such replacement of the LRU is resorted to only after time-consuming testing of the system has failed to further identify and pinpoint the exact nature of the failure within the LRU, the corrective action taken by the ground crew in replacing the unit is effective.
However, in a significant number of cases, the intermittent fault, which cannot be made to reoccur on the ground, is due to a failure not within an LRU, but rather is due to an intermittent defect in the interconnect wiring including the associated connectors that extend between the terminals in the equipment bay (or other central location) and the component of the LRU. One common type of fault of this nature occurs when a wire, or more probably a pin and associated socket in a multipin connector that forms part of the interconnect wiring, has an intermittent open. With existing BITEs and existing fault detection techniques which monitor the LRU component through the interconnect wiring, an intermittent open condition in one of the interconnect wires appears as an open component fault and as a consequence, the usual attempt to correct the problem is to replace the associated LRU. Of course, the corrective action is wholly ineffective, and when the fault reoccurs during flight, the entire maintenance procedure must be repeated, resulting in a large number of ineffectual maintenance actions. Studies have shown that intermittent connector failures in the interconnect wiring, where the failure appears as an open component in an LRU, are the cause underlying a majority of ineffective maintenance actions.
The frequency at which this type of intermittent fault occurs, is related, among other things, to the age of the aircraft, to the cable length over which the interconnect wiring is extended, and to the number of connectors that are used along any given run of cable. In some cases the interconnect wiring is extended to 100 feet or more. Also, numerous connectors are required in many cable runs for joining sections of cable that pass through partitions and other interior obstructions such as firewalls, and at joints between structural sections of the aircraft body, such as where a wing or other appendage joins the fuselage. As a result, some interconnect wiring, due to its length and number of intervening connectors, is prone to the type of open fault that is referred to above.
Although the foregoing maintenance problem is known to commercial airlines as well as aircraft manufacturers, a satisfactory solution has not been previously proposed. As mentioned, numerous BITE systems are available, however none have the capability of distinguishing faults in the interconnect wiring from faults in the LRU components, particularly where the components are of the type that are prone to open circuit faults, such as temperature and pressure sensors and the like. Components such as these are characterized by an impedance, lying somewhere between zero and infinity (open circuit) and the instantaneous impedance varies as a function of the parameter that is to be sensed.
One known BITE system that is pertinent to the present invention, is described in an article by J. R. Perkins, H. W. Heinzman and W. T. Turnage, "Proceedings of the National Aerospace Electronics Conference," 1973 record pages 355-359. In that article, a BITE system is disclosed in which a resistive impedance network is employed in conjunction with a switch signal source to enable the BITE to distinguish between an open wire fault and an open condition of the switch, and between a shorted interconnect wire and a closed condition of the same switch. However, it is noted that this known BITE system does not attempt (and is not capable of) detecting and distinguishing an electrical fault of the switch component (such as a stuck open failure of the switch) as well as a fault in the wiring. Also, this known BITE is employed in conjunction with a switch component that has discrete, open and closed impedance conditions of infinity and zero, respectively, and does not contemplate the use of fault prone components having predetermined intermediate impedance values or a range of continuously variable impedance levels, such as the above referred to discrete sensor components of certain LRUs.
Other heretofore proposed BITE systems require the use of active circuit elements directly in conjunction with the discrete component that is to be monitored. By active elements, it is meant that the fault detection of the BITE requires the injection of a separate voltage or current test signal that is independent of a control signal which is present as normal operating condition of the monitored component. Such active circuit BITE systems are less desirable than a passive BITE system, the latter of which uses the same voltage and/or current control signal that occurs normally during operation of the components that are being monitored. Thus a passive BITE circuit, as the term is used herein, does not require an additional, external electrical signal source that would add undue complexity and additional circuit elements that are themselves subject to possible malfunction.
Accordingly, it is an object of the invention to provide improved, passive fault detection circuitry for the electrical/electronic systems of an aircraft that remotely monitors the condition of individual, discrete elements of LRU components, such as temperature sensors, pressure sensors, valve solenoids and the like, by sensing operating signals associated with these elements as such signals appear at readily accessible terminals (such as in the electrical/electronic equipment bay), that are connected to the LRU components by interconnect wiring, and wherein the detection circuitry has the capability of distinguishing an open impedance fault of the component from an open impedance fault of the associated interconnect wiring.
Another object of the invention is to integrate such passive fault detection circuitry into the aircraft's electrical/electronic systems in a manner that enables the circuitry to monitor the condition of the discrete LRU components and associated interconnect wiring, at all times during their operation in the electrical system, so that intermittent faults, as well as steady faults, are detected during the flight. A related object is to provide as part of the fault detection circuitry, memory capability for storing the occurrence, location and nature of detected component and/or wiring faults so that intermittent faults are detected and their occurrence stored.
An additional object of the invention is to provide passive fault detection circuitry for the electrical/electronic systems of an aircraft, which has the foregoing capabilities, and which is cost effective in terms of the cost savings involved in eliminating unnecessary or ineffectual maintenance actions as compared to the cost of implementation and overall reliability and maintenance of the fault detection circuitry itself.